Stimuli-responsive and environmental-sensitive polymers and hydrogels [1-3] have been a subject of great interest due to their potential applications in, e.g. separation,[4] sensing [5, 6], catalysis [7], drug delivery [8, 9], and biomaterials [10]. When incorporated into a membrane, they can provide gating functions to the transport of ions and molecules by reversibly changing the permeability and selectivity of the membrane, as well as altering the adsorptive or hydrophilic/hydrophobic properties. The properties of these materials change in response to environmental changes or stimuli that can be mechanical, chemical, pH, temperature, redox potential, or light.
Recently, there has been increasing interest to apply stimuli-responsive polymers to energy storage systems, particularly in mitigating the undesirable and potentially dangerous effect of thermal runaway. For example, Roberts and coworkers investigated an electrolyte composed of thermally responsive copolymer of acrylic acid and poly(N-isopropylacrylamide) (pNIPAM). At low temperatures, pNIPAM is soluble, but phase segregates above the lower-critical solution temperature (LCST) and removes the ions from solution, thus causing a sharp decrease in the ionic conductivity of the electrolyte. In another example, Wei and coworkers used a coating of thermosensitive polymer P(N-isopropylacrylamide-co-2-acrylamido-2-methyl propane sulfonic acid) (P(NIPAM-co-SPMA)) to block access of ions to the pseudocapacitor material NiAl double layered hydroxide above the LCST.[11] The polymers in these two examples function in an aqueous system. Thus, they are limited to relatively low voltages.
Such aqueous systems are not suitable for Li ion batteries or super- and pseudocapacitors using ionic liquid or organic electrolytes, which are necessary for higher voltage applications. Thus far, no reversible thermal response systems have been reported for these non-aqueous applications. Currently for these systems, other means are employed to mitigate catastrophic thermal incidence. For example, measures used for Li ion batteries include introducing fire retardants or autonomous shut-down additives into the electrolyte,[12] applying positive temperature coefficient coatings on the electrode,[13, 14] or using shut-down separators.[15] However, once deployed, these processes are irreversible and the energy storage device is no longer functional.